As shown in FIG. 1, an aircraft wing 3 produces a vortex 6 at its tips. One simplified reason explaining the vortex production is illustrated in FIG. 2. The combination of (a) low pressure on the top of the wing, together with (b) higher pressure on the bottom of the wing, causes the air 9 to curl over as shown. The curling air becomes the vortex shown in FIG. 1.
Such a vortex represents a loss in energy, because the aircraft must expend energy in order to leave. behind the rotating vortex. In order to reduce this loss in energy, winglets 12 in FIG. 3 have been used. It is believed that the winglets act as fences which inhibit the curling action shown in FIG. 2.
Like a wing, a propeller 15 in FIG. 4 has a high-pressure surface and a low-pressure surface, and consequently produces a tip vortex (not shown). The tip vortex can produce noise. To reduce noise, it has been suggested that the tips 18 of the blades can be modified, as shown in FIGS. 4 and 4A.
A particular type of noise can occur in one type of propeller system. For example, in the counterrotating type, as shown in FIG. 5, the forward propeller 21 rotates in direction 23, while the aft propeller 25 rotates in direction 27. The vortices 30 produced by the forward propeller travel rearward, into the aft propeller. The aft propeller "chops" each vortex, producing noise.
One reason why the chopping causes noise is that the tip vortex changes the medium through which the propeller travels. The change causes the lift of the propeller blade to momentarily change, and noise results.
As a crude analogy, a ship's propeller 33 is shown in FIG. 6, and it is operating half-submerged in water. The propeller produces a thrashing noise because the blades alternate between two media, namely, air and water. In a roughly similar way, the aft propeller 25 in FIG. 5 produces noise when it chops a vortex 30.